Anthrax, a potentially fatal disease, is caused by Bacillus anthracis. The virulence of this pathogen is mediated by a capsule of a poly-D-γ-glutamic acid and an exotoxin composed of three proteins (14, 16, 17). The three protein components are the protective antigen (PA, 82 KDa), lethal factor (LF, 90.2 KDa) and edema factor (EF, 88.8 KDa) These proteins, non-toxic by themselves, form lethal toxins when combined with an activated PA (16). The genes coding for these three protein components and the capsule are found in the endogenous plasmids pXO1 and pXO2, respectively (29).
The capsule of Bacillus anthracis, composed of poly-D-glutamic acid, serves as one of the principal virulence factors during anthrax infection. By virtue of its negative charge, the capsule is purported to inhibit host defense through inhibition of phagocytosis of the vegetative cells by macrophages. In conjunction with lethal factor (LF) and edema factor (EF), whose target cells include macrophages and neutrophils, respectively, the capsule allows virulent anthrax bacilli to grow virtually unimpeded in the infected host. Spores germinating in the presence of serum and elevated CO2 release capsule through openings on the spore surface in the form of blebs which may coalesce before sloughing of the exosporium and outgrowth of the fully encapsulated vegetative cell. It has not been established that spore encapsulation plays a role in the early events of anthrax infection. The capsule appears exterior to the S-layer of the vegetative cell and does not require the S-layer for its attachment to the cell surface.
There is only indirect evidence, albeit extensive, identifying the components of vaccine-induced immunity to anthrax and there is evidence that anti-PA neutralizing antibody titers can be a reliable surrogate marker for protective immunity (23). The protective antigen (PA), seems to be an essential component of all vaccines for anthrax (7, 18, 30): both mono and polyclonal antibodies to PA neutralize the anthrax toxin and confer immunity to B. anthracis in animal models. The US licensed vaccine for anthrax “Anthrax Vaccine Adsorbed” (AVA) is produced from the formalin-treated culture supernatant of B. anthracis Sterne strain, V770-NP1-R (pXO1+, pXO2−), adsorbed onto aluminum hydroxide (22). Although AVA has been shown to be effective against cutaneous infection in animals and humans and against inhalation anthrax by rhesus monkeys (12), it has several limitations: 1) AVA elicits relatively high degree of local and systemic adverse reactions probably mediated by variable amounts of undefined bacterial products, making standardization difficult; 2) the immunization schedule requires administration of six doses within an eighteen-month period, followed by annual boosters for those at risk; and 3) there is no defined vaccine-induced protective level of serum PA to evaluate new lots of vaccines.
Development of a well characterized, standardized, effective and safe vaccine that would require fewer doses to confer immunity to both inhalational and cutaneous anthrax is needed (9, 30). It has been suggested that a vaccine composed of modified purified recombinant PA would be effective, safer, allow precise standardization, and probably would require fewer injections (27). Such a PA can be designed to be biologically inactive, more stable, and still maintained high immunogenicity.
In the examples herein, we describe the development of a production and purification process for recombinant PA from the non-sporogenic avirulent B. anthracis BH445 (pXO1−, pXO2−) strain. Following an 18-hour fermentation and three purification steps, large quantities of protective antigen suitable for vaccine production were obtained. The purified PA was tested in mice and was able to elicit neutralizing antibodies.